The development of cracking catalysts has heretofore generally been limited to the preparation of modified zeolites for use as cracking catalysts and to the interaction of such zeolites with other inorganic oxide materials. The following patents are representative of the prior art dealing with zeolite based cracking catalysts: The use of conversion catalysts formed from a zeolite dispersed in a siliceous matrix has been disclosed in U.S. Pat. No. 3,140,249 and U.S. Pat. No. 3,352,796. The use of blended matrix components, e.g., a catalyst comprising a zeolite, an inorganic oxide matrix and inert fines, which may be alpha alumina, is disclosed in U.S. Pat. No. 3,312,615. Catalysts comprising an amorphous silica-alumina, separately added alumina and a zeolite are disclosed in U.S. Pat. No. 3,542,670 and catalysts comprising a zeolite, an amorphous hydrous alumina and alumina monohydrate are disclosed in U.S. Pat. No. 3,428,550.
It has been disclosed that the steam and thermal stability of Y zeolites can be improved by the use of zeolites having a low level of alkali metal content and a unit cell size less than about 24.45 Angstroms; see U.S. Pat. Nos. 3,293,192 and Re. 28,629 (Reissue of U.S. Pat. No. 3,402,996). Further, it has been disclosed (U.S. Pat. No. 3,591,488) that the hydrogen or ammonium form of a zeolite may be treated with H.sub.2 O at a temperature ranging from about 800.degree. to about 1500.degree. F., and then subsequently cation exchanging the steam and water treated zeolite with cations, which may be rare earth metal cations. The method increases the silica to alumina mole ratio of the zeolite crystal framework and also the defect structure. U.S. Pat. No. 3,676,368 discloses a rare earth exchanged-hydrogen faujasite containing from 6 to 14 percent rare earth oxides. U.S. Pat. No. 3,957,623 discloses a rare earth exchanged zeolite having a total of 1 to 10 weight percent rare earth metal oxide. U.S. Pat. No. 3,607,043 discloses a process for preparing a zeolite having a rare earth content of 0.3 to 10 weight percent. U.S. Pat. No. 4,036,739 discloses hydrotherally stable and ammonia stable Y zeolite in which a sodium Y zeolite is ion exchanged to partially exchange sodium ions for ammonium ions, followed by steam calcination and a further ion exchange with ammonium to reduce the final sodium oxide content to below 1 weight percent, followed by calcination of the reexchanged product, or according to U.S. Pat. No. 3,781,199, the second calcinaticn may be conducted after the zeolite is admixed with a refractory oxide.
The above discussed prior art is representative of past and present day formulations of catalysts for fluid catalytic cracking (FCC). Recently a new class of materials was disclosed in U.S. Pat. No. 4,440,871. The materials of U.S. Pat. No. 4,440,871 are crystalline microporous silicoaluminophosphate ("SAPO") molecular sieves and are disclosed generally in columns 70 and 71 as employable in cracking processes as "SAPO compositions". Several of the "SAPOs" of U.S. Pat. No. 4,440,871 were evaluated by Lok for their catalytic cracking activity by use of an n-butane cracking test from which data a first-order rate constant was calculated. (See columns 72 and 73.) Although the first order rate constants for all the SAPO tested (SAPOs 5, 11, 17, 31, 34, 37 and 44) showed such to have catalytic activity, the rate constants varied from 0.2 to 7.4. Although Lok's n-butane cracking tests were carried out to provide "an indication of the catalytic cracking activity" of the SAPOs disclosed, it should be noted that these tests were conducted with fresh, unsteamed SAPOs. Assuming that cracking data for fresh, unsteamed SAPOs are pertinent to the actual performance of such materials in FCC processes, it might be predicted from Lok's data that catalysts based upon SAPO-5 would be at least as effective as catalysts based upon SAPO-37, and probably much better, since in the table in column 73 the n-butane cracking rate constant for SAPO-5 ranges from 1.4 to 7.4, while that for SAPO-37 ranges from 1.1 to 1.6. However, it is known that the cracking activity of fresh, unsteamed molecular sieves generally gives no indication of their utility in FCC processes, where commercial catalysts are subjected to very harsh hydrothermal environments in the regenerator section of the FCC unit. In such processes, the molecular sieve components of FCC catalysts typically lose significant proportions of their fresh catalytic activity within a short time in use; therefore molecular sieves should be evaluated for suitability for FCC catalyst use by their activity after steaming or similar hydrothermal treatments.
The use of a mixture of aluminosilicates and specific silicoaluminophosphates is disclosed in copending U.S. Ser. No. 935,599, a continuation of U.S. Ser. No. 675,285 which was filed concurrently herewith and commonly assigned.
Gasoline produced by fluid catalytic cracking (FCC) represents the largest blending component in the U.S. gasoline octane pool. According to a 1984 private study, FCC gasoline accounts for nearly 35 percent of all gasoline produced. FCC gasoline is a valuable blending component, since its octane rating {R+M}/2 of 86.5 to 87.5 was significantly above that of the pool at that time (85.9), thus increasing the pool's rating when mixed with other lower octane components. With the EPA-mandated lead phase-out scheduled for 1986, it was apparent that the U.S. gasoline octane pool rating must increase from 85.9 to greater than 88 to continue to meet automotive requirements without the use of lead. Several refinery processes including reforming, isomerization, alkylation and FCC have been used in efforts to meet this increased octane demand.
One way increased octane products can be obtained from the FCC reactor is through the use of specially designed FCC "octane" catalysts, which can increase RON by 3 to 4 numbers and MON by 1 to 1.5, as reported by J. S. Magee et al., in "Octane catalysts contain special sieves," Oil and Gas Journal, May 27, 1985, pp. 59-64. Octane enhancing catalysts introduced in the mid seventies include the Octacat.RTM. series by Davison, the Flexicat.RTM. series by Exxon, Engelhard's HFZ-20.RTM. and HFZ-33.RTM. series and more recently Katalistiks' Delta 400.RTM.. In early 1986, the usage of these octane catalysts was 50-60 tons/day, or approximately 10 percent of the U.S. market, and this has increased significantly after the lead phase-out began in 1986.
Almost all FCC cracking catalysts used commercially are based upon zeolite Y, which is known in the industry to be the most effective material presently available. Zeolite Y is apparently effective in FCC catalysis because it has a large pore, three dimensional channel system allowing rapid diffusion of reactants and products into and out of the zeolite. Zeolite Y also retains a significant proportions of its catalytic activity after destructive steam treatment, as it must in FCC processes.
Steam stabilized Y zeolite or ultrastable Y (USY) is believed to be the active component in most of these octane catalysts. When compared to calcined rare earth Y zeolite (CREY) catalysts, enhanced gasoline octane obtained using USY is accompanied by a reduction in coke make, which is a significant advantage to the refiner. USY's effect on gasoline selectivity is not as clear; some literature reports (Magee et al., supra) claim no change in gasoline selectivity relative to CREY, while others report a significant decrease in selectivity in the absence of rare earths (Pine et al., "Prediction of Cracking Catalysts Behavior by a Zeolite Unit Cell Size Model," J. Catalysis 85, 466, 1984). When gasoline yield loss is observed, it is accompanied by an increased gas make. In addition to the disadvantage of possibly reduced gasoline yields, it is clearly accepted that the USY based catalysts rapidly deactivate in steam and consequently are less active than CREY catalysts. Activity can be enhanced by using higher catalyst zeolite concentrations ("octane" catalysts contain up to 40 percent zeolite) but this is costly and results in reduced attrition resistance.
The mechanism by which USY-containing catalysts produce enhanced gasoline octane and decreased coke make is by now fairly well understood, as reported by Magee et al. and Pine et al., supra. Octane enhancement relative to CREY reportedly occurs because the gasoline produced over USY is significantly richer in olefins while somewhat lower in aromatics. Improved olefinicity is associated with reduced acid site concentration and lattice polarity for USY zeolites in FCC catalyst use, resulting in lower hydrogen transfer activity relative to CREY based catalysts. Hydrogen transfer consumes olefins and produces paraffins and aromatics. 3 olefins+1 naphthenes.fwdarw.3 paraffins +1 aromatic
Both olefins and aromatics are high octane gasoline components, but since three olefins are destroyed to produce one aromatic molecule a net octane loss results from this hydrogen transfer reaction. Further loss of hydrogen from hydrogen deficient products results in more paraffins and increased coke make. Literature reports reveal that the hydride reactions in FCC processes depend on the acid strength and acid site concentration and on the enhanced concentration of the reactant molecules in the Y zeolite (Rabo et al., "Zeolites in Industrial Catalysis," Acta Physica et Chemica, Nova Series, Hungaria, p. 39-52, 1976). The CREY catalyst provides the maximum acid site concentration as well as reactant concentrating ability, both of which lead to higher H.sup.- shift rates. In contrast, presteamed Y-82 and USY zeolites transform in use to LZ-10 type products, representing the extreme "low", both in acid site concentration and in reactant concentration, thus resulting in greatly reduced H.sup.- shift rates. Thus, USY FCC catalysts lacking this secondary hydride shift activity produce a more olefinic and higher octane gasoline than that produced over CREY catalysts.
The reasons for reduced gasoline selectivity observed over USY-containing catalysts have also been discussed in the literature. The olefinic gasoline from USY zeolite is more prone to secondary cracking reactions than the aromatic and paraffinic CREY gasoline. This results because it is easier to form carbonium ion intermediates from olefins than from paraffins. Carbonium ions are the high energy intermediates in cracking reactions. Secondary cracking reactions result in lower gasoline yield and higher gas make.
In summary then, there is a tradeoff in commercial catalytic cracking practice between high octane gasoline and high yields of gasoline. USY zeolite catalysts generally produce high octane gasoline in lower yield, while CREY catalysts produce higher yields of gasoline but at lower octane. The tradeoff is largely due to the presence of the secondary reactions of hydrogen transfer and cracking for the CREY and USY catalysts, respectively.
There is a need apparent in the industry to combine the desirable features of octane catalysts with those of the more extensively used CREY catalysts. Specifically, a target catalyst would exhibit the octane boost and coke selectivity associated with USY zeolites, while having the activity, stability and gasoline selectivity associated with CREY catalysts.
The instant invention relates to cracking catalysts and to catalytic cracking processes. The catalysts used comprise specific selected classes of silicoaluminophosphate molecular sieves disclosed in U.S. Pat. No. 4,440,871 having particular pore sizes and structures and are preferably employed with at least one inorganic oxide present as a binder and/or matrix component.